Geophysical modelling of structure and tectonostratigraphic history of the Longford Basin, Northern Tasmania

Abstract

A new gravity survey combined with ground-based magnetic surveys have enabled detailed modelling of basement structures in the Longford Basin of northern Tasmania. This basin began to form in the mid-Jurassic, but contains only Cainozoic sediments. The models suggest that the basin has been formed by a series of predominantly northwest-trending Jurassic normal faults which have produced a series of half-grabens in which blocks were downthrown to both east and west with dips of up to 10° W. The basin blocks incorporate Palaeozoic to Mesozoic Parmeener Supergroup sedimentary rocks intruded by pre- to syn-fault Jurassic dolerites. These units overlie an older, geophysically distinct, Devonian fold?thrust terrane. Much of the tectonic activity from the Palaeozoic to the Cainozoic appears to have been controlled by the Tiers Fault system, a major gravity and magnetic lineament within the Tasmanian crust. In contrast to many other Tasmanian Mesozoic?Cainozoic basins, the Longford Basin is exclusively continental, with up to 800 m of Paleocene to Quaternary fluvial sediments having been drilled in two wildcat oil wells. The basin also contains intercalated Late Eocene to Pliocene basaltic volcanic rocks. Magnetic data indicate three eruptive centres localised in reactivated Jurassic faults. Massive flows, now lateritised, have infilled an Early Eocene to Pliocene drainage system.